Effects of pressure on combustion dynamics in hydrogen micro-mixing flames

Annular micro-mixing (MM) nozzles anchor flames through multiple recirculation zones, showing high potential for stable combustion under varying operating conditions of gas turbines. However, their combustion dynamics under pressurized conditions remain unclear. This study experimentally investigates hydrogen flames in the annular MM nozzles, analyzing the flame time-frequency evolution, the relationship between the spatial distribution characteristics of heat-release pulsation frequencies/modes and the flame-stabilization zones, and the frequency-domain correspondence between pressure pulsations (P′) and heat-release pulsations at different inlet air pressures (P₃). The results show that, at P₃ = 0.1 MPa, the mean OH temporal distribution uniformity (TDU) is comparatively high. As P₃ increases from 0.2 MPa to 0.6 MPa, the mean OH TDU increases 12.8-fold, indicating intensified flame heat-release pulsations. At 0.5–0.6 MPa, the dominant frequencies of the flame heat-release pulsations become closer to the Helmholtz frequency of the combustor (≈92 Hz). At 0.1–0.6 MPa, the flame heat-release pulsations are dominated by the low-frequency mode around 100 Hz in the central regions of the nozzles, whereas the local high-frequency mode of 200–300 Hz in the inter-nozzle regions could hardly determine the overall flame heat-release pulsation behavior. The normalized RMS of pressure pulsations remains far below 1%, and the phase-space trajectories do not exhibit features of limit-cycle oscillations, indicating that the system does not enter a thermoacoustic instability state. Nevertheless, the energy proportion of the mode component in P′ that lies in the same frequency band as the dominant mode of heat-release pulsations rises by 142% with increasing P₃.